Chemical Genetic Analysis of Vertebrate Development Classic developmental genetics, involving large-scale mutagenesis screens, have shed much light on one of the most profound fundamental questions in biology: mechanistic insights into how a fertilized egg develops in an animal. Yet, because much of vertebrate development is inaccessible to traditional genetics, new approaches and tools are needed. As a complement to forward genetic analysis, we propose a large-scale chemical genetic analysis of early pattern formation in zebrafish embryos. Chemical genetic analysis, which involves the discovery and use of chemical probes, is a uniquely powerful approach to study vertebrate development. In a manner analogous to classic forward mutagenesis screens, a high-throughput chemical screen is conducted in zebrafish for small molecules that specifically modulate early embryonic development. Instead of mapping and cloning the responsible genes, the follow-up task involves identifying the pharmacological targets of the chemicals that elicit th prescribed phenotypic changes. As with forward genetic screens, the chemical genetic analysis can lead to the discovery of novel components or previously unanticipated signaling interactions involved. However importantly, since disturbances in developmental pathways are involved in the pathogenesis of many human illnesses, small molecules that selectively target them have significant translational and therapeutic potential. In the first specific aim, we will conduct a large-scale chemical screen for small molecules that specifically perturb early patterning in zebrafish embryos. As we have demonstrated, this is an innovative high content platform for discovering unique sets of highly selective modulators of developmental pathways. Next, we will employ chemical genetic analysis, which combines the methodologies of developmental biology, genetics, cell biology and pharmacology, to map the actions of small molecules discovered in Aim 1. This effort will be guided by the available molecular genetic information on early zebrafish embryogenesis. In addition, we propose a novel chemical genetic linkage analysis for target identification. We will combine lead optimization involving in vivo SAR (structure activity relationship) studies and target identification through large-scale drug target profiling. Finally, we propose an innovative platform for crowd sourcing of chemical genetic analyses and target identification efforts to overcome extant resource barriers that currently limi the impact of chemical genetics. In summary, the proposed study will discover valuable small molecules that selectively target key developmental pathways and use them to gain new insights on vertebrate development.